It's easy...she can't eat something that is not in the house, so if what she is snacking on are chips, crackers, cookies, anything processed, etc., do not buy them any more! Same goes for soda, "sports drinks" like Gatorade, Vitamin Water, Kool-Aid, etc. (when my stepsons were small, my biggest pet peeves were Pop Tarts and Lunchables - their mom would buy them for them but I refused).
If she must have chocolate milk, make it yourself and only add a very small amount of chocolate (my DD is fine if it's just barely got a hint of brown) - the stuff that's already pre-made often has as much sugar as soda. If she drinks juice, start watering it down. Make sure she is getting enough fresh water since sometimes being thirsty can feel like being hungry - you can add a squeeze of lemon or lime for flavor if needed. Or make ice cubes out of juice so again, they flavor the water, and the juice gets watered down.
If she needs to snack on something, provide fresh fruit (not canned), veggies like carrot and celery sticks, small numbers of nuts or raisins. Combinations of protein and whole grain carbs or fiber are good, like low-fat peanut butter with apple slices or low-fat cheese with Wheat Thins.
Buy the whole grain versions of bread, cereals, pastas, rice, etc. Make sure she is getting plenty of fiber, since this can help her feel fuller longer (again, fruits, veggies, whole grains, etc.). Make more things from scratch where you can alter the amount of sugar or fat in it, or eliminate it completely, like pumpkin muffins with whole wheat flour and oat flour. Make smoothies with low-fat yogurt, milk, and fruit. Milk should be 1% fat or less.
One rule of thumb for grocery shopping is to only buy foods with ingredients you can pronounce and don't sound like something out of a chemistry lab. If someone 100 years ago would not recognize it as food, it's probably not a good idea. Also, in a grocery store, the healthiest, most natural stuff is around the outside (fruits, veggies, etc.) - the bad stuff is usually lurking down the aisles.
If you just make some healthy changes at home, you don't need to talk to her about a "diet". And if she does indulge at her grandparents some, it's not the end of the world. But I would still talk to them and present like it's something her pediatrician and/or dentist recommended.
Get her involved in some physical activity, if possible. Find something she enjoys, or just try different things - soccer, swimming, dance, riding bikes, anything that gets her moving. And see what can be done to help alleviate any "boredom" if that is an issue - maybe the grandparents can play board games with her, rather than just letting her snack all the time. Or have the grandparents watch her at your house, so you are still in control of the food that is available.
There are on-line BMI calculators (body mass index) that can help determine if, based on height and weight and age, if she is possibly overweight or not. Some of it may be genetic, but that doesn't mean you can't make some changes now.